I've had my Betta for a few months now. He has been doing well in his 10 gallon, until recently when (Against my better judgement though finally being persuaded by other people) I decided to put cardinal tetras in with my Betta. Fuego went after them at first, but an hour or two later he gave up and they seemed to be fine. I kept checking up on them regularly and didn't see anything out of the ordinary. I did a 50% water change soon after and went to bed.
Woke up the next day and found two of the tetras dead, and the last had had its fins nibbled apart. Fuego seemed much more enthusiastic after I took the tetras out, as he had seemed pissed that I had the audacity to introduce new fish to his tank.
Anyway to get to the reason for my post, I went out shopping earlier and came back to find my fish nestled in the corner of my tank. I looked closer and noticed a white bacterial growth on his mouth, and he didn't seem as energetic as he had been earlier. I had noticed this white growth before, but didn't think too much of it (I'm an idiot) as I didn't see him acting in any way to indicate there was anything wrong with him.
I'll post pics up in a bit, but I'm going to run out to Petsmart in a bit to pick up a ph/nitrate chemical kit thing along with medicines. But the problem is I don't know what medicines to pick up and have zero experience with treating a sick fish. What type of things should I pick up to treat this? I'm going to do another water change when I get back, but for now I was just wondering what exactly I may need to cure my fishy.